Lithium ion batteries are usually designed with a pressure-limited valve and equipped with an external positive temperature coefficient (PTC) resistor on crust. However, these external devices may not be able to respond when hazardous reactions happen at very high rate.
There are several mechanisms to improve the safety of lithium ion battery under thermal runaway condition, such as thermal fuses, PTC materials as external or internal fuses and shutdown separators. However, none of them can achieve automatic shutdown of lithium ion batteries at a predetermined temperature.
Shutdown separators are designed to perform thermally-induced shutdown of lithium ion batteries, and typically have a polyethylene(PE)-polypropylene(PP) bilayer or a PP-PE-PP trilayer structure. Above a critical temperature, the porous PE layer softens, collapsing the film pores and preventing ionic conduction, while the PP layer provides mechanical support. However, when the internal cell temperature rises to the softening temperature of the separator, the separator shrinks because of residual stresses induced during stretching of the separator films and the difference in density between the crystalline and amorphous phases of the separator materials, which will cause failure of the separator and expose the electrodes to internal shorting.
Therefore, there is a need to improve safety of lithium ion batteries.